1. Field of the Invention
The present invention relates generally to a network using an Intermediate System to Intermediate System (IS--IS) protocol for connecting in a ring network; and more particularly, to a network element in such a network using the IS--IS protocol for connecting in such a ring network which may have at least one network element not using the IS--IS protocol.
2. Description of the Prior Art
The International Standards Organization (ISO) 10589 standard introduces the IS--IS protocol, which is planned for use in many network products worldwide. The IS--IS protocol gives a Network Element (NE) an automatic method of discovering the topology of a network. This automatic discovery, however, is restricted to NEs which support an IS--IS routing protocol. In order to know about NEs which do not support the IS--IS routing protocol, the IS--IS protocol allows end-users to manually provision information about these NEs.
For example, FIG. 1 shows a network having a first group with NEs 1, 2, and 10 that know about each other, and having a second group with NEs 5, 6 and 7 that know about each other (since they support IS--IS). However, the first and second groups of NEs do not know about the other group since they are connected to each other through NEs 3, 4, 8 and 9, which do not support the IS--IS routing protocol. To circumvent this problem, the IS--IS routing protocol allows manual information to be provisioned to NEs 2 and 10 about NEs 5, 6 and 7. The manual information also gets automatically propagated to NE 1. Similarly, NEs 7 and 5 can be provisioned with manual information about NEs 1, 2 and 10 (which gets distributed to NE 6 also). With the network provisioned in this manner, if NE 10 wants to send a packet to NE 6, it would know, via the manual information, that the way to get the packet there is by sending it to NE 9. NE 9 would send the packet to NE 8, which would send the packet to NE 7, and then from NE 7 to NE 6.
Now, if the link between NEs 8 and 9 were to break, NE 10 would not know about this, and for sending packets to NE 6, it would continue using the same route as before, even though this route is not valid anymore. Note that at this instant, there is an alternate route to NE 6, which NE 10 would not use since it thinks that the other route is better. The problem then is that since the IS--IS routing protocol does not have a mechanism for disabling/deleting invalid manual routing information, it can cause a lack of connectivity between two groups of NEs which could potentially still be connected.
This problem not only applies to link failures, but is applicable to network re-configuration situations also.